1. Field of the Invention
The present invention relates to a projection exposure mask (an original pattern plate), such as a photomask and a reticle for exposing a member under exposure to transfer a pattern thereon, a projection exposure apparatus, and a projection exposure method, each of which employs the mask.
2. Description of the Related Art
FIG. 18 shows a projection exposure apparatus for use in exposing a large substrate under exposure, such as a liquid crystal display panel, to transfer a circuit pattern thereon.
In FIG. 18, reference numeral 51 shows a mask, 52 a trapezoidal mirror, 53 a convex mirror, 54 a concave mirror, and 55 a substrate under exposure.
For transferring a circuit pattern to the substrate 55 in the exposure apparatus, exposure light L is irradiated to the mask 51, which corresponds to a negative film in photography. The exposure light L is transmitted through a pattern (a mask pattern) formed in the mask 51 via the trapezoidal mirror 52, the convex mirror 53, and the concave mirror 54, which serve as a projection system, and then forms the image of the mask pattern. The substrate 55 is disposed at the position where the mask pattern image is formed. In this manner, the circuit pattern is formed through exposure on the substrate 55.
The provision of a large-diameter projection system capable of exposing a large substrate under exposure to form a whole desired circuit pattern thereon at one time is disadvantageous in the area, weight, stability, and cost of the apparatus. Thus, a frequently employed projection system forms an image of part of a mask pattern in a slit-like shape, wherein a mask and a substrate under exposure are moved in synchronization with the projection system (driven for scanning) to scan an exposure light irradiation area of the mask and an area to be exposed on the substrate, thereby allowing the small projection system to form the image of the pattern through exposure on the large substrate.
In this case, the mask 51 and the substrate 55, which are sized to take account of the size of the mask pattern image formed on the substrate 55 and the projection magnification of the projection system, are relatively moved at a constant speed in the directions indicated by outline arrows in FIG. 18 with a controlled amount of exposure light to perform scan exposure. In the case of the exposure apparatus shown in FIG. 18, the mask 51 and the substrate 55 are mounted on a mask stage 57 and a substrate stage 56, respectively. The stages 57 and 56 are driven in the direction indicated by the outline arrows to scan the irradiation area of the exposure light L in the mask 51 and the area to be exposed on the substrate 55.
A number of projection exposure apparatuses which employ a transmission type mask have been commercially available as the apparatus shown in FIG. 18. In addition, a projection exposure apparatus proposed in Japanese Patent Application Laid-Open No. H11(1999)-219900 (corresponding to U.S. Pat. No. 6,359,678) employs a reflecting type mask for directing exposure light reflected by a mask pattern to a substrate under exposure and exposing the substrate.
The following problems are found in the scan type exposure apparatus in which the mask stage and the substrate stage are moved when the large substrate under exposure is exposed to form the circuit pattern thereon.
(1) As the substrate is increased in size, the mask is also increased in size and the manufacturing cost of the mask is increased.
(2) A larger mask produces a warp thereof due to its own weight in the exposure apparatus to cause difficulty in providing a required exposure resolution.
(3) The entire exposure apparatus is increased in size and weight.
The problem (1) is hereinafter described in detail. In exposure of a substrate, such as a liquid crystal display panel, circuit patterns for exposure include a continuous pattern such as a signal line and a gate line and a discontinuous cyclic pattern consisting of mutually isolated repetitive pattern elements, for example, a gate, a source, a drain, a transparent dot electrode, and a storage capacitor electrode. It is thus difficult to employ a so-called stitching exposure method in terms of formation of the continuous pattern. As a result, the exposure apparatus generally performs exposure at a projection magnification of 1:1 to cause an increased size of the mask in association with a larger size of the substrate for a liquid crystal display panel. This presents a significant problem in time and cost involved in the mask manufacture.
If the continuous pattern and the discontinuous cyclic pattern are processed in separate processes, the number of steps for exposure is increased to create disadvantages in process control and alignment, resulting in a factor which increases time and cost required for mask manufacture.
Next, the problem (2) is described in detail. The mask can be supported at its peripheries in the scan type exposure apparatus. A larger mask causes a warp thereof due to its own weight to use the margin of the focal depth of the projection system on the mask side. Thus, it is difficult to ensure a manufacture margin, such as flatness on the side of the substrate under exposure, leading to difficulty in providing a required exposure resolution.